The present invention relates generally to the field of audio processing and specifically to speech recognition using a microphone array.
As the popularity of wireless communications systems such as cellular telephones increases, many users utilize wireless mobile terminals in their automobiles. An increasingly popular trend is to equip mobile terminals with an external microphone and speaker, allowing for “hands-free” operation. In addition, it is known to include a speech recognition device in the hands-free adapter or the mobile terminal itself, so that the operator may, for example, “dial” a telephone number by voice command. While speech recognition technology is increasingly sophisticated, a clear separation of the voice component of an audio signal from noise components, i.e., a high Signal-to-Noise Ratio (SNR) is required for acceptable levels of accuracy in the speech recognition task. However, the movement of the microphone from adjacent the speaker's mouth, as in a hand-held unit, to a vehicle-mounted location, as in a hands-free unit, introduces significant noise into the audio input signal. Thus, a noise reduction operation must be performed on the audio signal prior to speech recognition to obtain satisfactory results.
Known audio noise reduction techniques, such as spectral subtraction, are deficient due to the frequency overlap between much motor vehicle cabin noise and the human speech spectrum. Additionally, frequency domain noise reduction techniques often introduce significant artifacts and aberrations into the speech audio component, making the speech recognition task more difficult.